1. Field of the Invention
The present invention relates to a microwave oven, and more particularly to the control of matching impedance of a microwave generating device and a load in a microwave oven.
2. Description of the Background Art
It is known that the impedance in a heating chamber varies depending upon the magnitude of load and position of the food item in a microwave oven. Because of this variance in impedance, the impedance between the magnetron and the heating chamber cannot be matched. As a result, the electric field by the microwave reflected from the heating chamber therein increases. In such a case, the power of the microwave cannot be effectively used. A microwave oven is disclosed that can adjust the position of a metal reflector for matching impedance in order to reduce the reflected electric field of the microwave (refer to Japanese Utility Model Laying-Open No. 61-100897, Japanese Patent Laying-Open No. 56-160792).
Another microwave oven is disclosed that can control impedance by moving a stub and/or a metal reflector depending upon the weight of food or the specified heating time according to a cooking program (refer to Japanese Utility Model Publication No. 1-25513, Japanese Patent Laying-Open No. 55-88289).
These conventional microwave ovens are directed to carry out effective operation of microwave power by closely controlling and adjusting the position of the device for matching impedance such as a metal reflector.
In order to effectively carry out impedance matching by rotating a metal reflector within a waveguide, it is necessary to move the position of a metal stub provided in a rotation plate (disk) of the metal reflector in the degree of .lambda.g/4 (.lambda.g: wavelength within waveguide of microwave) where the maximum and minimum value of the voltage of the standing wave of the microwave for the microwave oven (2450 MHz) appears.
Therefore, the rotation plate of the metal reflector must have a diameter of approximately 40 mm corresponding to .lambda.g/4. The device for matching impedance is provided in the waveguide having a high microwave electric field. In order to avoid arcing due to voltage induced by the microwave from the device for matching impedance of a metal reflector and the like, the device for matching impedance must be remote from the wall face of the waveguide by a sufficient spatial distance. Taking also into account the electric field concentration towards the end portion of the device for matching impedance, and change in the reflected wave depending upon the condition of load (light load) in the oven, the device for matching impedance should be as small as possible in order to avoid atmospheric discharge within the waveguide.
Effective usage of microwave power can be expected by controlling and adjusting the position of the impedance-matching reflector for reducing the reflected power from the oven including the substance to be heated. A possible method is to continuously control and drive the impedance-matching device by a motor or the like in cooking the substance-to-be-heated. However, because the load position is always changing in a microwave oven employing a turntable mechanism, the load is great in the driving mechanism for continuous follow. In view of the specified life time of a microwave oven, there is problem in the cost and reliability of the impedance-matching device.
In view of the foregoing, the usage of an impedance-matching device in a microwave oven manufactured in mass production has the following problems.
In the manufacturing stage, the structure of the impedance-matching device and the structure of members thereof must be made to be applicable for mass production while optimizing the adjustable region of impedance and carrying out measures with respect to arcing within the waveguide.
Although information of the food load in the oven is not obtained right after the initiation of a cooking operation, it is desirable to determine the operation mode of the impedance-matching device at the start of the operation to obtain a preferable result.
Although the heated time of cooking is normally short in cooking light food the time for detecting information to determine the heating mode in operation of the food in the oven by an electric field sensor or a weight sensor takes several seconds to ten several seconds which is a relatively long time. Because the heating mode at the start of operation greatly affects the heating efficiency of food of light load, the load must be determined taking into consideration the magnitude thereof.
A change in impedance is expected due to difference in temperature before and after cooking the food item, or due to change in weight caused by vaporization of the food. It is therefore necessary to sense and correct it appropriately.
The impedance-matching device employed in an economic type microwave oven for domestic use must have low component cost.